Leach can be more effective than archery. It depends mainly on where you are placing the towers. In hillside, demons, and original, there are areas where you can place leaches that get 2 hits as the blazes circle them. For leaches this means a very extended period to charge. If you are slowing the blazes in this region as well, and assuming the enemy does not speed them, leaches can be very effective. As soon as the first leach charges, the blazes are temporarily stunned, letting other leach towers charge further, until all have fully charged and dealt considerable damage. However, remember this method can be quite expensive, costing around 8000 coins. I would only advise this method if you are someone who rushes the last goldmine.
For most of the time, I would advise archery towers, especially if you are at 15 cps to 25 cps, which every player on the team should have acquired within around 5 minutes. However be warned that you will need a very large amount of them, especially if the enemy speeds the blazes. In the case of enemy speeding, you will need almost half your track covered in archery (which is why I prefer large maps), and you will need to drop slowness potions, along with a considerable amount of meteor shower (NEVER use zeus's rage or inferno on blazes. Zeus's rage should only be used on giant and wither, and inferno almost never. Meteor shower is probably the best defensive potion). Also, when building a huge archery range, most people upgrade all the towers to lvl 4. However, this is a huge waste of money. You should only ever upgrade your archery to lvl 3, unless in a map like perilous, where there are 3 rows and you need the range. Lvl 4 costs 60 coins and only increases the range of the archery tower by 1 block, nothing else. So if you have a range of say, 40 archery towers, you could have saved yourself 2400 coins, enough to buy over 340 lvl 1 archery towers!
However, in the end I would say leach is the best tower to place. Also, in the original question, it seemed you wanted to know whether you should unlock leach. To anyone else with the same question, I was say yes for sure, unless you do not currently have necromancer tower unlocked, in which that becomes a priority. The leach tower is not only good in the above set up for defeating blazes, it is also excellent for giants. My personal strategy is to have 3-5 necromancer towers positioned fairly close to start, a little ways before the leaches. These necros are almost always enough to split the giant, but then you are left with magma cubes. You how have 3 options. 1. Destroy the magma cubes with meteor shower (this takes a while and is very expensive but it is possible, I'd advise having the last goldmine). 2. Destroy the magma cubes with artillery (with the assistance of quake/ice) towers. However, this usually takes a lot of artillery, and by the time the giant was split, it is past your start artillery range already, so you would need a large second one, space that could be used for archery for the blazes. 3. Leach towers. I discovered this method accidentally when trying to beat blazes, and it works quite well. With slowness potions, the magma cubes (already slower) barely crawl along the track. When they reach the leaches, each tower charges 2-3 times between their slow movement and stuns from leaches. After the leaches are finished with the magma cubes, you'll have 3-4 tiny cubes, which can be taken care of with just a couple artillery, or even your sword.
NOTE: Be warned, the above method with leaches creates considerable lag. Make sure you don't get kicked from lag and lose the game that way :D